The molecular mechanisms of opportunistic infection by Toxoplasma gondii in AIDS patients will be investigated. Central nervous system toxoplasmosis has caused disability and death in up to 30% of patients with AIDS. Current clinical treatment for toxoplasma infection is mainly chemotherapeutic, which is often associated with dose-related bone marrow suppression. Complete remission of the infection depends largely on the adequate restoration of the cell-mediated immune system of the patients and thus, it poses a continuous threat to AIDS patients. An alternative treatment using recombinant interferon-gamma (IFN-gamma) has been proposed since IFN-gamma, a CD4+ T cell product, blocks the growth of T. gondii in human epithelial and peripheral blood mononuclear cells, and macrophages. Treatment of mice with monoclonal anti-IFN-gamma antibody abrogates resistance to toxoplasma infection. The IFN-gamma-induced anti-toxoplasmic activity in human cells is strongly correlated with the degradation of the essential amino acid L-tryptophan in vitro. Destruction of L-tryptophan is due to increased activity of indoleamine 2,3-dioxygenase (IDO) which is in turn transcriptionally activated by IFN-gamma. It has been suggested that T. gondii infection is primarily controlled by IDO activity induced by IFN-gamma derived from CD4+ T cells. Therefore, we propose the following four specific aims to investigate the role of IDO in host defense against T. gondii infection. (1) To determine whether IDO activity alone, expressed via stable transfection of IDO cDNA controlled by a metallothionein inducible promoter, blocks the growth of T. gondii in cultured human fibroblast and macrophage cell lines; (2) To investigate whether IDO activity expressed in brain or hematopoietic system of mice via gene transfer approaches would confer resistance to T gondii infection in mice; (3) To examine the levels of IDO expression in mice either vaccinated with an avirulent strain of T. gondii or vaccinated but depleted of IFN-gamma through specific antibody injection; (4) Since the mechanisms of macrophage inhibition of parasite replication are unclear, to examine the role of IDO against T. gondii replication by asking if IDO activity shows an inverse correlation with the parasite growth in hosts depleted of CD4+ T cells and if macrophage activators (or deactivators), other than IFN-gamma, induce IDO and anti-toxoplasmic activity. The long term objective of this proposed research is to provide molecular information used for developing new and effective drugs for the prevention and treatment of toxoplasmosis in AIDS patients.